Lecture 15:Meteorites and Cosmic Collisions Claire Max May 21st, 2009 Astro 18: Planets and Planetary Systems UC Santa Cruz
Practicalities • During Break: • Take a look at our meteorites • Next Tuesday May 26: • Come to class with your project groups, work on projects together • We will take attendance: you must come! • Thursday May 28th: • Final lecture: Life in the Universe
Outline of lecture • Meteorites: • How are meteorites found? • Main types • Where do they come from? • Meteorites as time capsules • Cosmic Collisions • Role of cosmic collisions in evolution of Solar System • History of collisions • Collision of Comet Shoemaker-Levy 9 with Jupiter • Effects of impacts • Prospects for future giant collisions with Earth Please remind me to take a break at 2:45 pm
The main points: Meteorites • Each year the Earth sweeps up ~80,000 tons of extraterrestrial matter, from microscopic dust particles to large rocks • Some are identifiable pieces of the Moon, Mars, or Vesta; most are pieces of asteroids • Meteorites were broken off their parent bodies 10’s to 100’s of million years ago (recently compared to age of Solar System) • Oldest meteorites (chondrites) contain bits of interstellar dust, tiny diamonds made in supernova explosions, organic molecules and amino acids (building blocks of life), tiny spherules left over from the very early Solar System • Direct insight into pre-solar system matter, solar system formation
Meteor showers • Time exposure image, tracking stellar motion • Stars stay still, meteorites make trails
Rocks Falling from the Sky • meteor – a flash of light caused by a particle which enters Earth’s atmosphere. • most of these particles are the size of a pea or smaller • they completely burn up in Earth’s atmosphere • meteorite – a rock which is large enough to have survived its fall to Earth • they caused a brighter meteor…sometimes a fireball • How can you tell that you have a meteorite? • they have a higher metal content than terrestrial rocks • they contain Iridium and other isotopes not found in terrestrial rocks
What are meteorites? • Chunks of rock or iron-nickel that fall to Earth from space • Pieces of asteroids, comets, Moon, Mars, interstellar dust • Can weigh from < 1 ounce to a few tons (!) • “The Poor Man’s Space Probe” • From parts of the Solar System astronauts may never explore • Usually named after the place where they fall • Examples: Prairie Dog Creek (US), Zagora (Morocco), Campo del Cielo (Argentina), Mundrabilla (Australia)
What do meteorites look like? Vesta Mars meteorite Allen Hills (Moon)
Variety of meteorite “falls” • Tiny pieces of cosmic dust • Collected by special airplanes, in clay under the oceans, or in Antarctic ice • Find single small chunks of rock • Sometimes at random, sometimes by following trajectory of a “fireball” or meteor trail • A several-ton meteorite breaks up during descent, falls as separate pieces • Biggest pieces can make large craters if they hit land
Small particles: spherules • Tiny droplets from space • Formed by melting and re-solidification after impacts Spherule from Moon Collected by Apollo 11 astronauts Spherule from bottom of the Indian Ocean
Small particles: cosmic dust • Sometimes from comets, sometimes left over from the cosmic dust cloud from which the Solar System formed
Single small chunks of rock Iron-nickel meteorite A few inches across Allende Carbonaceous chondrite
Several-ton boulders Hoba Meteorite, Namibia
Tonguska meteorite in Siberia caused widespread devastation • Fortunately it hit in an unpopulated area!
How meteorites are found • Random “finds” lying on ground • Fragments around meteor craters • Follow glowing trail of meteor or fireball • Systematic searches in Antarctica • Special high-flying airplanes (for dust)
Random “finds” • Rare: a big meteorite in desert of Oman • Pretty rare: random “finds” of smaller chunks
Fragments around meteor craters Barringer Crater, Arizona • Very large meteorites vaporize when they hit ground, form big craters • Sometimes small pieces are found around crater
This year in Sudan.... • Link to Scientific American article
University of Khartoum students did systematic search • 45 students and staff of the University of Khartoum rode buses out to desert, searched in lines. Found more than 280 pieces
Macroscopic features of the Almahata Sitta meteorite. P Jenniskens et al.Nature458, 485-488 (2009)
primitive about 4.6 billion years old accreted in the Solar nebula processed younger than 4.6 billion years matter has differentiated fragments of a larger object which processed the original Solar nebula material Primitive vs. processed meteorites Based on composition, meteorites fall into two basic categories:
Origin of Meteorites • Primitive meteorites condensed and accreted directly from the Solar nebula. • the stony ones formed closer than 3 AU from the Sun • the Carbon-rich ones formed beyond 3 AU from the Sun, where it was cold enough for Carbon compounds to condense • Processed meteorites come from large objects in the inner Solar System. • the metallic ones are fragments of the cores of asteroids which were shattered in collisions • the rocky ones were chipped off the surfaces of asteroids, Mars, and the Moon by impacts
Main types of meteorites • Chondrites • Carbonaceous • Non-carbonaceous • Achondrites • Iron • Stony-Iron
Chondrites • Rocky, inhomogeneous, contain round “chondrules”
Carbonaceous Chondrites contain complex organic molecules • Amino acids, fatty acids, other so-called “building blocks of life” • Did building blocks of life come to Earth from space? • Did life itself come to Earth from space? • “Panspermia” theory
Carbonaceous Chondrites: Insights into Planet Formation? • The oldest meteorites; quite rare • Chondrules (round): primitive chunks of early Solar System • Calcium aluminum inclusions (CaI’s): isotope ratios (26 Al and 26 Mg) suggest that a supernova explosion went off right next to the early Solar Nebula • Did the supernova stimulate formation of our Solar System?
Iron meteorites • Made of iron and nickel • Pits made during atmospheric entry (hot!)
Crystalization pattern of the iron is unique • Characteristic of very slow cooling of iron within an asteroid core • Due to diffusion of nickel atoms into solid iron as core cools • Says original asteroid must have been large enough to be differentiated
Stony-Iron meteorites - the prettiest • Crystals of olivene (a rock mineral) embedded in iron • From boundary between core and mantel of large asteroids?
Achondrites: from Mars and Moon • From Mars: • Tiny inclusions have same elements and isotope ratios as Martian atmosphere (measured by spacecraft on Mars) • From the Moon: • Astronauts brought back rocks from several regions on the Moon • Some achondrites match these rock types exactly
Where do meteorites come from, and how do we know? • Spectra: reflection of sunlight as function of wavelength of light • Spectra of some meteorites and asteroids can be identical • Implies asteroid was parent body Toro
The main points: Meteorites • Each year the Earth sweeps up ~80,000 tons of extraterrestrial matter • Some are identifiable pieces of the Moon, Mars, or Vesta; most are pieces of asteroids • Meteorites were broken off their parent bodies 10’s to 100’s of million years ago (recently compared to age of Solar System) • Oldest meteorites (chondrites) contain interstellar dust, tiny diamonds made in supernova explosions, organic molecules and amino acids (building blocks of life) • Direct insight into pre-solar system matter, solar system formation
The main points: Cosmic Collisions • Cosmic collisions played major role in Solar System evolution • Aggregation of planets from planetesimals • Formation of Moon, tilt of Venus’ and Uranus’ rotation axes, composition of Mercury • Also played a major role in Earth’s evolution • Tilt of axis • Mass extinctions (dinosaurs, others) • Collision history derived from crater patterns, isotope ratios • Probability of global catastrophic impact event once every 100 million years • Strong interest in tracking all Near-Earth Objects (NEO’s) that might hit the Earth in the future
Role of cosmic collisions in evolution of Solar System • Early phase (4.5 billion yrs ago): planet formation • Planetesimals collided or accreted to form larger pieces • Formation of Moon by glancing collision with Earth • Removal of most of Mercury’s crust by collision • Collision made Venus rotate backwards • Collision tipped Uranus onto its side (now rotates at 90 deg to rotation axes of all other planets) • “Late Heavy Bombardment” (~3.9 billion years ago) from Lunar record • First signs of life on Earth immediately followed “Late Heavy Bombardment” period. Is there some sort of causal connection?
Evidence that Moon formed as result of a collision • Earth has large iron core, but the moon does not • Earth's iron had already drained into the core by the time of the giant impact that formed the moon • Debris blown out of both Earth and the impactor came from their iron-depleted, rocky mantles • Explains why mean density of Moon (3.3 grams/cm3) is much less than Earth (5.5 grams/cm3) • Moon has same oxygen isotope composition as the Earth • Mars and meteorites from outer Solar System have different oxygen isotope compositions • Moon formed form material formed in Earth's neighborhood.
Formation of the Moon…. • Large planetesimal collides w/ Earth at glancing angle • Removed material is from mantle of Earth
Uranus’ rotation axis lies in plane of its orbit • Unique in Solar System • All other planets’ rotation axes point out of the plane of their orbits
Collision with a massive body is best way to explain this • Would have to have collided with a body at least as big as the Earth • Approached Uranus at a large angle to the plane of the Solar System